I want to have a Parrty but dont know if i can anymore :/?

Just try talking about the situation with her, but stay calm! If you get all hyped up and act immature, she'll just blow off whatever you're saying. Tell her why you'd really like to have it in Friday (because everyone believes that it's on friday, and you can't change plans at the last minute like this...plus, ppl are busy on Saturday). Also, tell her that having the party a day early doesn't matter, because it's just a party...not changing your age. Tell her just to think of it as a party..not as a bday thing.